Christos Yiallouras

Review of MRI positioning devices for guiding focused ultrasound systems

This article contains a review of positioning devices that are currently used in the area of magnetic resonance imaging (MRI) guided focused ultrasound surgery (MRgFUS).

Three-axis MR-conditional robot for high-intensity focused ultrasound for treating prostate diseases transrectally

BackgroundA prototype magnetic resonance image (MRI)-conditional robot was developed for navigating a high-intensity focused ultrasound (HIFU) system in order to treat prostate cancer transrectally.Materials and methodsThe developed robotic device utilizes three PC-controlled axes: a linear axis for motion along the rectum, an angular axis for rotation in the rectum, and a linear axis to lift the robot up and down. Experiments with the system were performed in a 1.5-T MRI system using gel phantoms.ResultThe robot was successfully operated in a 1.5-T clinical MRI system. The effect of piezoelectric motors and optical encoders was quantified based on the reduction of signal to noise ratio. Discrete and overlapping lesions were created accurately by moving the HIFU transducer with the robotic device.ConclusionAn MRI-conditional HIFU robot was developed which can create controlled thermal lesions under MRI guidance. The intention is to use this robot transrectally in the future for the treatment of prostate cancer.

The Enhancing Effect of Focused Ultrasound on TNK-Tissue Plasminogen Activator-Induced Thrombolysis Using an In Vitro Circulating Flow Model

BACKGROUND The limited efficacy of thrombolytic therapy in patients with ischemic stroke has created the need to use focused ultrasound (FUS) energy as a way to enhance thrombolysis efficacy (sonothrombolysis). Using an in vitro circulating flow model, we evaluated the role of physical parameters on tenecteplase (TNK-tPA)-mediated thrombolysis. MATERIALS AND METHODS Fully retracted porcine blood clots were used for the proposed experimental study. To provide a more realistic clinical environment of stroke, the study was conducted under realistic flow conditions and TNK-tPA concentrations. Two spherically FUS transducers (4-cm diameter), focusing at 10 cm and operating at .6 and 1.05 MHz, respectively, were used. Pulsed ultrasound protocols that maintained a localized temperature elevation at the focus of 1°C were applied. Thrombolysis efficacy was measured in milligram of mass clot removed. RESULTS The effect of physical parameters such as temperature, FUS frequency, acoustic power (AP), FUS energy, and microbubble (MB) administration on thrombolysis efficacy was examined. CONCLUSIONS Study findings established that higher FUS frequencies (1 MHz) are associated with enhanced thrombolysis compared to lower FUS frequencies (.6 MHz). Furthermore, an increase in the linear relationship between AP and thrombolysis efficacy was exhibited. Also, the outcome of the study showed that the combination of 1-MHz FUS pulses with MBs strongly enhanced the enzymatic thrombolytic efficacy of TNK-tPA, because with 30 minutes of treatment, 1050 mg of clot was removed through nonthermal mechanisms. Taking into consideration that stroke is time dependent, this thrombolytic rate should be sufficient for timely recanalization of the occluded cerebral artery.

MRI guided focused ultrasound robotic system for the treatment of gynaecological tumors.

A novel MRI‐conditional robot that navigates focused ultrasound (FUS) for the treatment of gynaecological tumors endovaginally was developed.

MRI-compatible bone phantom for evaluating ultrasonic thermal exposures.

OBJECTIVE The goal of the proposed study was the development of a magnetic resonance imaging (MRI) compatible bone phantom suitable for evaluating focused ultrasound protocols. MATERIALS AND METHODS High resolution CT images were used to segment femur bone. The segmented model was manufactured with (Acrylonitrile Butadiene Styrene) ABS plastic using a 3-D printer. The surrounding skeletal muscle tissue was mimicked using an agar-silica-evaporated milk gel (2% w/v-2% w/v-40% v/v). MR thermometry was used to evaluate the exposures of the bone phantom to focused ultrasound. RESULTS The estimated agar-silica-evaporated milk gels T1 and T2 relaxation times in a 1.5T magnetic field were 776ms and 66ms respectively. MR thermometry maps indicated increased temperature adjacent to the bone, which was also shown in situations of real bone/tissue interfaces. CONCLUSION Due to growing interest of using MRI guided Focused Ultrasound Surgery (MRgFUS) in palliating bone cancer patients at terminal stages of the disease, the proposed bone phantom can be utilized as a very useful tool for evaluating ultrasonic protocols, thus minimizing the need for animal models. The estimated temperature measured and its distribution near the bone phantom/agar interface which was similar to temperatures recorded in real bone ablation with FUS, confirmed the phantoms functionality.

Evaluation of focused ultrasound algorithms: Issues for reducing pre-focal heating and treatment time

BACKGROUND Due to the heating in the pre-focal field the delay between successive movements in high intensity focused ultrasound (HIFU) are sometimes as long as 60s, resulting to treatment time in the order of 2-3h. Because there is generally a requirement to reduce treatment time, we were motivated to explore alternative transducer motion algorithms in order to reduce pre-focal heating and treatment time. MATERIALS AND METHODS A 1 MHz single element transducer with 4 cm diameter and 10 cm focal length was used. A simulation model was developed that estimates the temperature, thermal dose and lesion development in the pre-focal field. The simulated temperature history that was combined with the motion algorithms produced thermal maps in the pre-focal region. Polyacrylimde gel phantom was used to evaluate the induced pre-focal heating for each motion algorithm used, and also was used to assess the accuracy of the simulation model. RESULTS Three out of the six algorithms having successive steps close to each other, exhibited severe heating in the pre-focal field. Minimal heating was produced with the algorithms having successive steps apart from each other (square, square spiral and random). The last three algorithms were improved further (with small cost in time), thus eliminating completely the pre-focal heating and reducing substantially the treatment time as compared to traditional algorithms. CONCLUSIONS Out of the six algorithms, 3 were successful in eliminating the pre-focal heating completely. Because these 3 algorithms required no delay between successive movements (except in the last part of the motion), the treatment time was reduced by 93%. Therefore, it will be possible in the future, to achieve treatment time of focused ultrasound therapies shorter than 30 min. The rate of ablated volume achieved with one of the proposed algorithms was 71 cm(3)/h. The intention of this pilot study was to demonstrate that the navigation algorithms play the most important role in reducing pre-focal heating. By evaluating in the future, all commercially available geometries, it will be possible to reduce the treatment time, for thermal ablation protocols intended for oncological targets.

An MRI‐conditional motion phantom for the evaluation of high‐intensity focused ultrasound protocols

The respiratory motion of abdominal organs is a serious obstacle in high‐intensity focused ultrasound (HIFU) treatment with magnetic resonance imaging (MRI) guidance. In this study, a two‐dimensional (2D) MRI‐conditional motion phantom device was developed in order to evaluate HIFU protocols in synchronized and non‐synchronized ablation of moving targets.

MRI‐compatible breast/rib phantom for evaluating ultrasonic thermal exposures

The target of this study was the development of a magnetic resonance imaging (MRI) compatible breast phantom for focused ultrasound which includes plastic (ABS) ribs. The objective of the current study was the evaluation of a focused ultrasound procedure using the proposed phantom that eliminates rib heating.

MRI-guided frameless biopsy robotic system with the inclusion of unfocused ultrasound transducer for brain cancer ablation

A magnetic resonance image (MRI) guided robotic system dedicated for brain biopsy was developed. The robotic system carries a biopsy needle and a small rectangular unfocused, single element, planar ultrasonic transducer which can be potentially utilized to ablate small and localized brain cancer.

The role of three-dimensional printing in magnetic resonance imaging-guided focused ultrasound surgery

Objectives: This article describes novel magnetic resonance imaging (MRI)-compatible focused ultrasound robotic systems and agar-based MRI-compatible ultrasonic phantoms mimicking bone. Materials and Methods: All the robotic systems and phantoms were developed using three-dimensional (3D) printing technology using plastic material. The tissue surrounding the bone in the phantoms was mimicked using agar-based solutions. Results: The article presents MRI-guided focused ultrasound robotic systems for brain, prostate, and gynecological targets. It also reports on MRI-compatible ultrasonic phantoms for brain, breast, bone, and motion. Conclusions: The popular 3D printing technology serves a major role in MRI-guided focused ultrasound surgery because MRI-guided focused ultrasound robotic systems can be developed. In addition, 3D printing can be used to develop MR-compatible phantoms that include bone structures for testing the safety and efficacy of focused ultrasound applications. All the developed structures have been evaluated in MRI environment using either mimicking materials or animals.